Multiple pulse defibrillation for subcutaneous implantable cardiac devices

ABSTRACT

Cardiac stimulation methods and systems provide for multiple pulse defibrillation, and involve sensing a fibrillation event, determining a fibrillation cycle length associated with the fibrillation event, and delivering a plurality of defibrillation pulses to treat the fibrillation event. Defibrillation pulses are delivered using a combination of subcutaneous non-intrathoracic electrodes. Delivery of each defibrillation waveform subsequent to a first defibrillation waveform is separated in time by a delay associated with the fibrillation cycle length. Delays between defibrillation waveform delivery may be associated with a percentage of the fibrillation cycle length.

FIELD OF THE INVENTION

The present invention relates generally to implantable medical devicesand, more particularly, to subcutaneous cardiac sensing and stimulationdevices employing delivery of multiple electrical defibrillation pulseswith timing based on the sensed cardiac activity.

BACKGROUND OF THE INVENTION

The healthy heart produces regular, synchronized contractions. Rhythmiccontractions of the heart are normally initiated by the sinoatrial (SA)node, which is a group of specialized cells located in the upper rightatrium. The SA node is the normal pacemaker of the heart, typicallyinitiating 60-100 heartbeats per minute. When the SA node is pacing theheart normally, the heart is said to be in normal sinus rhythm.

If the heart's electrical activity becomes uncoordinated or irregular,the heart is denoted to be arrhythmic. Cardiac arrhythmia impairscardiac efficiency and may be a potential life-threatening event.Cardiac arrhythmias have a number of etiological sources, includingtissue damage due to myocardial infarction, infection, or degradation ofthe heart's ability to generate or synchronize the electrical impulsesthat coordinate contractions.

Bradycardia occurs when the heart rhythm is too slow. This condition maybe caused, for example, by impaired function of the SA node, denotedsick sinus syndrome, or by delayed propagation or blockage of theelectrical impulse between the atria and ventricles. Bradycardiaproduces a heart rate that is too slow to maintain adequate circulation.

When the heart rate is too rapid, the condition is denoted tachycardia.Tachycardia may have its origin in either the atria or the ventricles.Tachycardias occurring in the atria of the heart, for example, includeatrial fibrillation and atrial flutter. Both conditions arecharacterized by rapid contractions of the atria. Besides beinghemodynamically inefficient, the rapid contractions of the atria mayalso adversely affect the ventricular rate.

Ventricular tachycardia occurs, for example, when electrical activityarises in the ventricular myocardium at a rate more rapid than thenormal sinus rhythm. Ventricular tachycardia may quickly degenerate intoventricular fibrillation. Ventricular fibrillation is a conditiondenoted by extremely rapid, uncoordinated electrical activity within theventricular tissue. The rapid and erratic excitation of the ventriculartissue prevents synchronized contractions and impairs the heart'sability to effectively pump blood to the body, which is a fatalcondition unless the heart is returned to sinus rhythm within a fewminutes.

Implantable cardiac rhythm management systems have been used as aneffective treatment for patients with serious arrhythmias. These systemstypically include one or more leads and circuitry to sense signals fromone or more interior and/or exterior surfaces of the heart. Such systemsalso include circuitry for generating electrical pulses that are appliedto cardiac tissue at one or more interior and/or exterior surfaces ofthe heart. For example, leads extending into the patient's heart areconnected to electrodes that contact the myocardium for sensing theheart's electrical signals and for delivering pulses to the heart inaccordance with various therapies for treating arrhythmias.

Typical implantable cardioverter/defibrillators include one or moreendocardial leads to which at least one defibrillation electrode isconnected. Such implantable cardioverter/defibrillators are capable ofdelivering high-energy shocks to the heart, interrupting the ventriculartachyarrhythmia or ventricular fibrillation, and allowing the heart toresume normal sinus rhythm.

SUMMARY OF THE INVENTION

The present invention is directed to cardiac stimulation methods andsystems that provide for multiple pulse defibrillation. A method ofdelivering a subcutaneous defibrillation therapy in accordance with thepresent invention involves sensing a fibrillation event, determining afibrillation cycle length associated with the fibrillation event, anddelivering two or more defibrillation pulses to treat the fibrillationevent. Delivery of each pulse subsequent to the first pulse is separatedin time by a delay associated with the fibrillation cycle length.

Embodiments of the present invention deliver the defibrillation pulsesfrom one or more subcutaneous non-intrathoracic electrode arrangements.Multiple defibrillation pulses may be delivered following a delayassociated with a percentage of the fibrillation cycle length. The delaymay be a fixed percentage of the cycle length, or may be a variablepercentage of the cycle length. Three or more defibrillation pulses maybe delivered with fixed or variable delays, one or more of which may beassociated with a percentage of the fibrillation cycle length.

Delivery of the defibrillation pulses may be facilitated using the sameor different electrode arrangements/combination of electrodes. Sensingdefibrillation events may be facilitated using the same or differentelectrode arrangements/combination of electrodes. Embodiments of thepresent invention may provide for delays between defibrillation pulsesgreater than about 50 percent of the fibrillation cycle length, greaterthan about 75 percent of the fibrillation cycle length, and betweenabout 75 to 100 percent of the fibrillation cycle length, respectively.The fibrillation cycle length may be determined using varioustechniques, including a Fourier analysis of the fibrillation event, anauto-correlation analysis of the fibrillation event, and/or by countingsignal features of the fibrillation event over a duration of time.

Defibrillation pulses may be substantially similar in terms of one ormore of amplitude, duration, and phase, or may be substantiallydifferent in terms of one or more of amplitude, duration, and phase.Defibrillation pulses may include monophasic pulses, biphasic pulses, ora combination of monophasic and biphasic pulses.

A system for delivering a subcutaneous defibrillation therapy inaccordance with the present invention includes a housing configured forsubcutaneous non-intrathoracic placement relative to a patient's heart.Detection circuitry may be provided in the housing, the detectioncircuitry configured to detect a fibrillation event. Energy deliverycircuitry may also be provided in the housing, and coupled to one ormore electrode arrangements configured for subcutaneousnon-intrathoracic placement relative to the patient's heart. Acontroller may further be provided in the housing and coupled to thedetection and energy delivery circuitry, the controller configured todetermine a fibrillation cycle length associated with the fibrillationevent. The controller may coordinate delivery of defibrillation pulsesusing a delay between pulses associated with a fixed or variablepercentage of the fibrillation cycle length.

The controller may be further configured to deliver three or more pulsesin a multiple pulse defibrillation waveform, where the pulses may beseparated by delays of the same or differing lengths. One or more of thedelays may be associated with a percentage of the fibrillation cyclelength. One or more electrode arrangements/electrode combinations may beused for detection of the fibrillation event and delivery of thedefibrillation pulses. For example, a first electrode arrangement orcombination may be used for detection of the fibrillation event and asecond electrode arrangement or combination may be used for delivery ofthe defibrillation pulses. Other embodiments include a first electrodearrangement/combination used for delivery of an initial pulse and asecond electrode arrangement/combination used for delivery of one ormore subsequent pulses. Any combination of electrodes may be used fordelivery of any of the multiple defibrillation pulses without departingfrom the scope of the present invention.

The above summary of the present invention is not intended to describeeach embodiment or every implementation of the present invention.Advantages and attainments, together with a more complete understandingof the invention, will become apparent and appreciated by referring tothe following detailed description and claims taken in conjunction withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of three pulses of differing energies that may be usedto describe cardiac defibrillation levels;

FIG. 2 is a graph of three pulses of differing durations that may beused to describe cardiac defibrillation timing;

FIG. 3 is an illustration depicting the average cycle length of acardiac ventricular tachycardia and/or ventricular fibrillation episode;

FIG. 4 is a graph depicting regions of separation and interaction formultiple pulse defibrillation therapy in accordance with the presentinvention;

FIG. 5 is a graph of energy delivery versus defibrillation shockduration developed from single shock and two shock multiple pulsedefibrillation studies performed in accordance with the presentinvention;

FIG. 6 is a top view of an implantable cardiac device in accordance withthe present invention, including an antenna electrode and a lead/headerarrangement;

FIG. 7 is a diagram illustrating components of a cardiac stimulationdevice including an electrode array in accordance with an embodiment ofthe present invention;

FIG. 8 is a block diagram illustrating various components of a cardiacstimulation device in accordance with an embodiment of the presentinvention; and

FIG. 9 is a block diagram of a medical system that may be used toimplement system updating, coordinated patient monitoring, diagnosis,and/or therapy in accordance with embodiments of the present invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail below. It is to be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the invention isintended to cover all modifications, equivalents, and alternativesfalling within the scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

In the following description of the illustrated embodiments, referencesare made to the accompanying drawings, which form a part hereof, and inwhich is shown by way of illustration, various embodiments in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized, and structural and functional changes maybe made without departing from the scope of the present invention.

An implanted device according to the present invention may include oneor more of the features, structures, methods, or combinations thereofdescribed hereinbelow. For example, a cardiac stimulator may beimplemented to include one or more of the advantageous features and/orprocesses described below. It is intended that such a stimulator, orother implanted or partially implanted device need not include all ofthe features described herein, but may be implemented to includeselected features that provide for useful structures and/orfunctionality. Such a device may be implemented to provide a variety offunctions.

A wide variety of implantable cardiac stimulation devices may beconfigured to implement a multiple pulse defibrillation methodology ofthe present invention. A non-limiting, representative list of suchdevices includes defibrillators, cardioverters, resynchronizers, andother cardiac therapy delivery devices. These devices may be configuredwith a variety of electrode arrangements, including transvenous,endocardial, and epicardial electrodes (i.e., intrathoracic electrodes),and/or subcutaneous, non-intrathoracic electrodes, including can,header, and indifferent electrodes, and subcutaneous array or leadelectrodes (i.e., non-intrathoracic electrodes). In general, however, amultiple pulse defibrillation methodology of the present invention ispreferably implemented using subcutaneous, non-intrathoracic electrodes.Embodiments of the present invention are referred to herein generally aspatient-internal medical devices (PIMD) for convenience. A PIMDimplemented in accordance with the present invention may incorporate oneor more of the electrode types identified above and/or combinationsthereof.

FIG. 1 is a graph of three pulses that may be used to describe cardiacdefibrillation energies. The ordinate of the graph illustrated in FIG. 1is time, and the abscissa is current amplitude, indicative of thecurrent delivered by a cardiac defibrillator. A plot 110 illustrates anormalized square pulse at a current 112 and a duration 114. The energyassociated with the plot 110 is designated E. A plot 120 is illustratedas having a current amplitude 122 twice that of the current amplitude112 of plot 110, but having a duration 124, which is the same asduration 114 of plot 110. Due to the square law for energy, the energyof the pulse of plot 120 is four times the energy of the pulse in plot110, or 4E. A plot 130 is illustrated as having a current amplitude 132,which is the same as that of the current amplitude 112 of plot 110, butat a duration 134 that is twice the duration 114 of plot 110. The energyof the pulse of plot 130 is only twice that of the pulse in plot 110(2E), since the energy square law does not apply in this case. Inapplications where energy is potentially limited, such as forimplantable devices using batteries for power, pulses of longer durationare more efficient than pulses of larger current, assuming the desiredtherapy objective may be achieved using either pulse type.

FIG. 2 is a graph of three pulses of differing durations that may beused to describe cardiac defibrillation timing. The ordinate of thegraph illustrated in FIG. 2 is time, and the abscissa is currentamplitude, indicative of the current delivered by a cardiacdefibrillator. A plot 210 illustrates a bipolar pulse having a period212 designated by T. In accordance with the present invention, it ispossible to achieve defibrillation, at similar energy, by splitting apulse of period 212 into two pulses of shorter period (e.g., T/2),separated by a delay in delivery timing 226. For example, a 2-pulsesignal 220 is separated by delivery timing delay 226. Each of a pulse222 and a pulse 224 has a period of T/2, thereby maintaining similarenergies, such that the pulse of plot 210 has an energy approximatelyequal to that of the multiple pulse of 2-pulse signal 220. The exampleof splitting the 2-pulse signal 220 into T/2 durations at similarcurrent amplitudes is for illustrative purposes only, and not forpurposes of limitation.

The delivery timing 226 plays an important role in the efficacy ofdefibrillation for use in multiple pulse defibrillation systems andmethods. Referring now to FIG. 3, a graph 300 depicts an average cyclelength 310 associated with a cardiac ventricular tachycardia and/orventricular fibrillation episode. The signal of graph 300 may beacquired using a cardiac electrode arrangement to measure theelectrophysiological signals produced by a patient's heart during aventricular tachycardia and/or ventricular fibrillation episode. Bydetermining the period of oscillation of the ventricular tachycardiaand/or ventricular fibrillation episode, the average cycle length 310may be defined. For example, zero crossings may be determined for thesignal 300, and used to determine the average cycle length 310. Othermethods for determining average cycle length include, for example,auto-correlation techniques, Fourier transform methods, and countingother morphological features, such as maximas, minimas, inflectionpoints, or other features.

Delivering multiple defibrillation pulses separated by a delay indelivery timing relative to the average cycle length of the ventriculartachycardia and/or ventricular fibrillation provides defibrillationefficacy at a defibrillation energy similar to that of a single pulse.FIG. 4 is a graph depicting regions of separation and interaction formultiple pulse defibrillation in accordance with the present invention.

A region 320 exists at about a delivery delay up to about 35% of theaverage cycle length. The region 320 is considered to be the regionwhere the delay between the multiple pulses is so short relative to asingle pulse, that the cardiac response to the delay is imperceptible.

A region 330 exists at a delivery delay from about 35% to about 50% ofthe average cycle length, wherein the cardiac response to the separatedpulses is less efficacious than a single pulse. The region 330 may beconsidered as a region where destructive interference occurs between thecardiac response to the first pulse and the cardiac response to asubsequent pulse. Because of the region 330, delays greater than about50% of the average cycle length may provide improved efficacy formultiple pulse defibrillation.

A region 350 exists at delay times greater than about 125% of theaverage cycle length, where the pulses act independently. Since multiplepulses are delivered with each pulse occurring during a separatefibrillation cycle, significant interactions are not obtained, andimprovements in efficacy are not observed. In view of the region 350,delays less than about 125% of the average cycle length may provideimproved efficacy for multiple pulse defibrillation.

However, there exists a region 340, at a multiple pulse timing delaybetween about 50% and about 125%, and typically between about 75% andabout 100% of the average cycle length, where the cardiac response tomultiple separated pulses is similar to the cardiac response to a singlepulse. This region, which may be considered similar to a region ofconstructive interference for the cardiac response to the separatedpulses, provides opportunity for improved efficacy of defibrillation,decreased energy requirements for defibrillation systems, or bothimproved efficacy and decreased energy.

For example, a single pulse at current I and duration T may be replacedwith two pulses at current I and duration T/2, separated by a delay of85% of the average cycle length. This multiple pulse defibrillationtherapy may have similar or better cardiac response than the singlepulse, but save ⅓^(rd) the energy. By way of further example, a singlepulse at current I and duration T (=24 milliseconds) may be replacedwith two pulses at current 0.81×I and duration T/2 (=12 milliseconds),separated by a delay of 85% of the average cycle length. The multiplepulse defibrillation at a decreased current and an increased durationmay provide a significantly better cardiac response than a single pulseusing the same energy. As will now be appreciated by those skilled inthe art, energy may be advantageously adjusted, such as by selecting anumber of pulses, pulse widths, and amplitudes, to provide a desiredlevel of defibrillation efficacy and energy conservation in accordancewith the present invention.

In order to illustrate advantages of multiple pulse defibrillationmethods of the present invention, preclinical studies were performed oncanine subjects. FIG. 5 is a graph 500 comparing single shock controlgroup and two shock multiple pulse defibrillation group preclinicalstudies performed in accordance with the present invention. The ordinateof the graph illustrated in FIG. 5 is the total duration ofdefibrillation shock, and the abscissa is normalized (I50) current,indicative of the current delivered by a cardiac defibrillator necessaryto defibrillate 50 percent of the test subjects.

The graph 500 includes defibrillation measures in the same caninesubjects with a single shock control 510, and a two shock multiple pulsedefibrillation 520. Graph 500 also includes an estimation plot 530 ofthe (I50) current necessary to achieve equal energy as the single shockcontrol group 510. The multiple pulse defibrillation 520 exhibited moreeffective defibrillation at less current than the control 510particularly at longer total shock durations. Graph 500 demonstratesthat a multiple pulse defibrillation methodology in accordance with thepresent invention provides an opportunity for a 20% decrease indefibrillation current to achieve similar results to the control groupin a PIMD.

The ability to defibrillate more effectively using multiple pulsedefibrillation in accordance with the present invention permitsdefibrillation with longer shock durations and lower shock currents.This is particularly useful for subcutaneous implantable cardiac deviceswhere shock electrodes are not situated near, or in direct contact, withthe cardiac tissue. In such electrode configurations, the shock currentfrom the defibrillation electrodes must distribute throughout the entirethorax so that only a relatively small fraction of that current passesdirectly through the cardiac tissue to achieve defibrillation.

For instance, say that a subcutaneous single shock with a12-milliseconds duration delivered a current (Ic) to the cardiac tissue.To achieve this current in the cardiac tissue, a current of (F×I) mustbe applied to the subcutaneous defibrillation electrodes where (1/F) isthe fraction of that applied current that passes directly through thecardiac tissue for defibrillation. Now, if successful defibrillationrequired that the cardiac tissue current be (Id) for a single12-milliseconds shock, then the current at the subcutaneous electrodeswould need to be increased to (F×I×Id/Ic) and the energy would need tobe increased by a factor of (Id²/Ic²). On the other hand, doubling thetotal shock duration to 24 milliseconds by multiple pulse defibrillationin accordance with the present invention, would permit successfuldefibrillation with an applied current of (F×I×0.74×Id/Ic) and theenergy would be increased by a factor of (0.54×Id²/Ic²). Thus, comparedto increasing the intensity of a single shock to achieve subcutaneousdefibrillation, doubling its total duration by multiple pulsedefibrillation in accordance with the present invention will reduce therequired defibrillation energy by a factor of (0.54) while at the sametime reducing the required current by a factor of 0.74. In cases where Fis on the order of 5 to 10, such as in subcutaneous defibrillation, thisis particularly important since it limits the maximum current requiredof the subcutaneous defibrillator.

For purposes of illustration, and not of limitation, various embodimentsof devices that may implement multiple pulse defibrillation inaccordance with the present invention are described herein in thecontext of PIMDs that may be implanted under the skin in the chestregion of a patient. A PIMD may, for example, be implantedsubcutaneously such that all or selected elements of the device arepositioned on the patient's front, back, side, or other body locationssuitable for sensing cardiac activity and/or delivering cardiacstimulation therapy. It is understood that elements of the PIMD may belocated at several different body locations, such as in the chest,abdominal, or subclavian region with electrode elements respectivelypositioned at different regions near, around, in, or on the heart.

The primary housing (e.g., the active or non-active can) of the PIMD,for example, may be configured for positioning outside of the rib cageat an intercostal or subcostal location, within the abdomen, or in theupper chest region (e.g., subclavian location, such as above the thirdrib). In one implementation, one or more leads incorporating electrodesmay be located in direct contact with the heart, great vessel, orcoronary vasculature, such as via one or more leads implanted by use ofconventional transvenous delivery approaches. In another implementation,one or more electrodes may be located on the primary housing and/or atother locations about, but not in direct contact with the heart, greatvessel, or coronary vasculature.

In a further implementation, for example, one or more electrodesubsystems or electrode arrays may be used to sense cardiac activity anddeliver cardiac stimulation energy in a PIMD configuration employing anactive can or a configuration employing a non-active can. Electrodes maybe situated at anterior and/or posterior locations relative to theheart. Examples of useful electrode locations and features that may beincorporated in various embodiments of the present invention aredescribed in commonly owned, co-pending U.S. patent application Ser. No.10/465,520 filed Jun. 19, 2003, now U.S. Publication No. 2004/0230230;Ser. No. 10/795,126 filed Mar. 5, 2004, now U.S. Publication No.2005/0197674; Ser. No. 10/738,608 filed Dec. 17, 2003, now U.S.Publication No. 2004/0230243; Ser. No. 10/821,248 filed Apr. 8, 2004,now U.S. Publication No. 2004/0215240; Ser. No. 10/785,431 filed Feb.24, 2004, now U.S. Publication No. 2005/0004615; and Ser. No. 10/462,001filed Jun. 13, 2003, now U.S. Publication No. 2004/0230229; which arehereby incorporated herein by reference.

Certain configurations illustrated herein are generally described ascapable of implementing various functions traditionally performed by animplantable cardioverter/defibrillator (ICD), and may operate innumerous cardioversion/defibrillation modes as are known in the art.Examples of ICD circuitry, structures, and functionality, aspects ofwhich may be incorporated in a PIMD of a type that may benefit frommultiple pulse defibrillation methods and implementations are disclosedin commonly owned U.S. Pat. Nos. 4,996,984; 5,133,353; 5,161,528;5,179,945; 5,314,459; 5,318,597; 5,620,466; and 5,662,688, which arehereby incorporated herein by reference.

A PIMD in accordance with the present invention may implement diagnosticand/or monitoring functions as well as provide cardiac stimulationtherapy. Examples of cardiac monitoring circuitry, structures, andfunctionality, aspects of which may be incorporated in a PIMD of a typethat may benefit from multiple pulse defibrillation methods andimplementations are disclosed in commonly owned U.S. Pat. Nos.5,313,953; 5,388,578; and 5,411,031, which are hereby incorporatedherein by reference.

Various embodiments described herein may be used in connection withcongestive heart failure (CHF) monitoring, diagnosis, and/or therapy. APIMD of the present invention may Incorporate CHF features involvingdual-chamber or bi-ventricular pacing/therapy, cardiac resynchronizationtherapy, cardiac function optimization, or other CHF relatedmethodologies. For example, any PIMD of the present invention mayincorporate features of one or more of the following references:commonly owned U.S. patent application Ser. No. 10/270,035, filed Oct.11, 2002, entitled “Timing Cycles for Synchronized Multisite CardiacPacing;”, now U.S. Publication No. 2003/0130702, and U.S. Pat. Nos.6,411,848; 6,285,907; 4,928,688; 6,459,929; 5,334,222; 6,026,320;6,371,922; 6,597,951; 6,424,865; and 6,542,775, each of which is herebyincorporated herein by reference.

A PIMD may be used to implement various diagnostic functions, which mayinvolve performing rate-based, pattern and rate-based, and/ormorphological tachyarrhythmia discrimination analyses. Subcutaneous,cutaneous, and/or external sensors may be employed to acquirephysiologic and non-physiologic information for purposes of enhancingtachyarrhythmia detection and termination. It is understood thatconfigurations, features, and combination of features described in thepresent disclosure may be implemented in a wide range of implantablemedical devices, and that such embodiments and features are not limitedto the particular devices described herein.

FIG. 6 is a top view of a PIMD 782 in accordance with the presentinvention, having at least two electrodes. One electrode is illustratedas an antenna 705 of the PIMD that may also be used for RFcommunications. The PIMD 782 shown in the embodiment illustrated in FIG.6 includes a first electrode 798 and a second electrode 799 coupled to acan 703 through a header 789, via an electrode module 796. The firstelectrode 798 and second electrode 799 may be located on a lead 783(single or multiple lead, or electrode array), or may be locateddirectly in or on the electrode module 796.

The can 703 is illustrated as incorporating the header 789. The header789 may be configured to facilitate removable attachment between anelectrode module 796 and the can 703, as is shown in the embodimentdepicted in FIG. 6. The header 789 includes a female coupler 792configured to accept a male coupler 793 from the electrode module 796.The male coupler 793 is shown having two electrode contacts 794, 795 forcoupling one or more electrodes 798, 799 through the electrode module796 to the can 703. An electrode 781 h and an electrode 781 k areillustrated on the header 789 of the can 703 and may also be coupledthrough the electrode module 796 to the can 703. The can 703 mayalternatively, or in addition to the header electrodes 781 h, 781 kand/or first and second electrodes 798, 799, include one or more canelectrodes 781 a, 781 b, 781 c.

Electrodes may also be provided on the back of the can 703, typicallythe side facing externally relative to the patient after implantation.For example, electrodes 781 m, 781 p, and 781 r are illustrated aspositioned in or on the back of the can 703. Providing electrodes onboth front and back surfaces of the can 703 provides for athree-dimensional spatial distribution of the electrodes, which mayprovide additional discrimination capabilities. Further description ofthree-dimensional configurations are described in U.S. patentapplication Ser. No. 10/795,126 entitled “Wireless ECG In ImplantableDevices,”, now U.S. Publication No. 2005/0197674, previouslyincorporated by reference.

In this and other configurations, the header 789 incorporates interfacefeatures (e.g., electrical connectors, ports, engagement features, andthe like) that facilitate electrical connectivity with one or more leadand/or sensor systems, lead and/or sensor modules, and electrodes. Theheader 789 may also incorporate one or more electrodes in addition to,or instead of, the electrodes provided by the lead 783, such aselectrodes 781 h and 781 k, to provide more available vectors to thePIMD. The interface features of the header 789 may be protected frombody fluids using known techniques.

The PIMD 782 may further include one or more sensors in or on the can703, header 789, electrode module 796, or lead(s) that couple to theheader 789 or electrode module 796. Useful sensors may includeelectrophysiologic and non-electrophysiologic sensors, such as anacoustic sensor, an impedance sensor, a blood sensor, such as an oxygensaturation sensor (oximeter or plethysmographic sensor), a bloodpressure sensor, minute ventilation sensor, or other sensor described orincorporated herein.

In one configuration, as is illustrated in FIG. 7, electrode subsystemsof a PIMD system are arranged about a patient's heart 810. The PIMDsystem includes a first electrode subsystem, including a can electrode802, and a second electrode subsystem 804 that includes at least twoelectrodes or at least one multi-element electrode. The second electrodesubsystem 804 may include a number of electrodes used for sensing and/orelectrical stimulation.

In various configurations, the second electrode subsystem 804 mayinclude a combination of electrodes. The combination of electrodes ofthe second electrode subsystem 804 may include coil electrodes, tipelectrodes, ring electrodes, multi-element coils, spiral coils, spiralcoils mounted on non-conductive backing, screen patch electrodes,subcutaneous arrays, and other electrode configurations. A suitablenon-conductive backing material is silicone rubber, for example.

The can electrode 802 is positioned on the housing 801 that encloses thePIMD electronics. In one embodiment, the can electrode 802 includes theentirety of the external surface of the housing 801. In otherembodiments, various portions of the housing 801 may be electricallyisolated from the can electrode 802 or from tissue. For example, theactive area of the can electrode 802 may include all or a portion ofeither the anterior or posterior surface of the housing 801 to directcurrent flow in a manner advantageous for cardiac sensing and/orstimulation.

Portions of the housing may be electrically isolated from tissue tooptimally direct current flow. For example, portions of the housing 801may be covered with a non-conductive, or otherwise electricallyresistive, material to direct current flow. Suitable non-conductivematerial coatings include those formed from silicone rubber,polyurethane, or parylene, for example.

FIG. 8 is a block diagram depicting various componentry of differentarrangements of a PIMD in accordance with embodiments of the presentinvention. The components, functionality, and configurations depicted inFIG. 8 are intended to provide an understanding of various features andcombinations of features that may be incorporated in a PIMD. It isunderstood that a wide variety of device configurations arecontemplated, ranging from relatively sophisticated to relatively simpledesigns. As such, particular PIMD configurations may include somecomponentry illustrated in FIG. 8, while excluding other componentryillustrated in FIG. 8.

Illustrated in FIG. 8 is a processor-based control system 905, whichincludes a micro-processor 906 coupled to appropriate memory (volatileand/or non-volatile) 909, it being understood that any logic-basedcontrol architecture may be used. The control system 905 is coupled tocircuitry and components to sense, detect, and analyze electricalsignals produced by the heart and deliver electrical stimulation energyto the heart under predetermined conditions to treat cardiacarrhythmias. The control system 905 and associated components alsoprovide pacing therapy to the heart. The electrical energy delivered bythe PIMD may be in the form of low energy pacing pulses or high-energypulses for cardioversion or defibrillation.

Cardiac signals are sensed using the electrode(s) 914 and the can orindifferent electrode 907 provided on the PIMD housing. Cardiac signalsmay also be sensed using only the electrode(s) 914, such as in anon-active can configuration. As such, unipolar, bipolar, or combinedunipolar/bipolar electrode configurations as well as multi-elementelectrodes, subcutaneous arrays, and combinations of noise canceling andstandard electrodes may be employed. The sensed cardiac signals arereceived by sensing circuitry 904, which includes sense amplificationcircuitry and may also include filtering circuitry and ananalog-to-digital (A/D) converter. The sensed cardiac signals processedby the sensing circuitry 904 may be received by noise reductioncircuitry 903, which may further reduce noise before signals are sent tothe detection circuitry 902.

Detection circuitry 902 may include a signal processor that coordinatesanalysis of the sensed cardiac signals and/or other sensor inputs todetect cardiac arrhythmias, such as, in particular, tachyarrhythmia.Rate based and/or morphological discrimination algorithms may beimplemented by the signal processor of the detection circuitry 902 todetect and verify the presence and severity of an arrhythmic episode.

The detection circuitry 902 communicates cardiac signal information tothe control system 905. Memory circuitry 909 of the control system 905contains parameters for operating in various sensing, defibrillation,and, if applicable, pacing modes, and stores data indicative of cardiacsignals received by the detection circuitry 902. The memory circuitry909 may also be configured to store historical ECG and therapy data,which may be used for various purposes and transmitted to an externalreceiving device as needed or desired.

In certain configurations, the PIMD may include diagnostics circuitry910. The diagnostics circuitry 910 typically receives input signals fromthe detection circuitry 902 and the sensing circuitry 904. Thediagnostics circuitry 910 provides diagnostics data to the controlsystem 905, it being understood that the control system 905 mayincorporate all or part of the diagnostics circuitry 910 or itsfunctionality. The control system 905 may store and use informationprovided by the diagnostics circuitry 910 for a variety of diagnosticspurposes. This diagnostic information may be stored, for example,subsequent to a triggering event or at predetermined intervals, and mayinclude system diagnostics, such as power source status, therapydelivery history, and/or patient diagnostics. The diagnostic informationmay take the form of electrical signals or other sensor data acquiredimmediately prior to therapy delivery.

The control system 905 processes cardiac signal data received from thedetection circuitry 902 and initiates appropriate tachyarrhythmiatherapies to terminate cardiac arrhythmic episodes and return the heartto normal sinus rhythm. The control system 905 is coupled to shocktherapy circuitry 916. The shock therapy circuitry 916 is coupled to theelectrode(s) 914 and the can or indifferent electrode 907 of the PIMDhousing.

Upon command, the shock therapy circuitry 916 delivers multiple pulsedefibrillation stimulation energy to the heart in accordance with thepresent invention. In a more sophisticated configuration, the shocktherapy circuitry 916 is controlled to deliver defibrillation therapiesand cardioversion therapies. Examples of PIMD high energy deliverycircuitry, structures, and functionality, aspects of which may beincorporated in a PIMD of a type that may benefit from aspects of thepresent invention are disclosed in commonly owned U.S. Pat. Nos.5,372,606; 5,411,525; 5,468,254; and 5,634,938, which are herebyincorporated herein by reference.

Arrhythmic episodes may also be detected and verified bymorphology-based analysis of sensed cardiac signals as is known in theart. Tiered or parallel arrhythmia discrimination algorithms may also beimplemented using both rate-based and morphologic-based approaches.Further, a rate and pattern-based arrhythmia detection anddiscrimination approach may be employed to detect and/or verifyarrhythmic episodes, such as the approach disclosed in U.S. Pat. Nos.6,487,443; 6,259,947; 6,141,581; 5,855,593; and 5,545,186, which arehereby incorporated herein by reference.

In accordance with another configuration, a PIMD may incorporate acardiac pacing capability in addition to multiple pulse defibrillationcapabilities. As is shown in FIG. 8, the PIMD includes pacing therapycircuitry 930 that is coupled to the control system 905 and theelectrode(s) 914 and can/indifferent electrodes 907. Upon command, thepacing therapy circuitry 930 delivers pacing pulses to the heart inaccordance with a selected pacing therapy.

The PIMD shown in FIG. 8 may be configured to receive signals from oneor more physiologic and/or non-physiologic sensors. Depending on thetype of sensor employed, signals generated by the sensors may becommunicated to transducer circuitry coupled directly to the detectioncircuitry 902 or indirectly via the sensing circuitry 904. It is notedthat certain sensors may transmit sense data to the control system 905without processing by the detection circuitry 902.

Communications circuitry 918 is coupled to the microprocessor 906 of thecontrol system 905. The communications circuitry 918 allows the PIMD tocommunicate with one or more receiving devices or systems situatedexternal to the PIMD. It is noted that physiologic or non-physiologicsensors equipped with wireless transmitters or transceivers maycommunicate with a receiving system external of the patient.

The communications circuitry 918 allows the PIMD to communicate with anexternal programmer. In one configuration, the communications circuitry918 and the programmer unit (not shown) use a wire loop antenna and aradio frequency telemetric link, as is known in the art, to receive andtransmit signals and data between the programmer unit and communicationscircuitry 918. In this manner, programming commands and data aretransferred between the PIMD and the programmer unit during and afterimplant. Using a programmer, a physician is able to set or modifyvarious parameters used by the PIMD. For example, a physician may set ormodify parameters affecting sensing, detection, pacing, anddefibrillation functions of the PIMD, including pacing andcardioversion/defibrillation therapy modes.

Typically, the PIMD is encased and hermetically sealed in a housingsuitable for implanting in a human body as is known in the art. Power tothe PIMD is supplied by an electrochemical power source 920 housedwithin the PIMD.

The detection circuitry 902, which is coupled to a microprocessor 906,may be configured to incorporate, or communicate with, specializedcircuitry for processing sensed cardiac signals in manners particularlyuseful in a cardiac sensing and/or stimulation device. As is shown byway of example in FIG. 8, the detection circuitry 902 may receiveinformation from multiple physiologic and non-physiologic sensors.

The components, functionality, and structural configurations depictedherein are intended to provide an understanding of various features andcombination of features that may be incorporated in a PIMD. It isunderstood that a wide variety of PIMDs and other implantable cardiacstimulation device configurations are contemplated, ranging fromrelatively sophisticated to relatively simple designs. As such,particular PIMD or cardiac stimulation device configurations may includeparticular features as described herein, while other such deviceconfigurations may exclude particular features described herein.

Referring now to FIG. 9, a PIMD of the present invention may be usedwithin the structure of an advanced patient management (APM) system1000. The advanced patient management system 1000 allows physicians toremotely and automatically monitor cardiac and respiratory functions, aswell as other patient conditions. In one example, a PIMD implemented asa cardiac defibrillator may be equipped with various telecommunicationsand information technologies that enable real-time data collection,diagnosis, and treatment of the patient. Various PIMD embodimentsdescribed herein may be used in connection with advanced patientmanagement. Methods, structures, and/or techniques described herein,which may be adapted to provide for remote patient/device monitoring,diagnosis, therapy, or other APM related methodologies, may incorporatefeatures of one or more of the following references: U.S. Pat. Nos.6,221,011; 6,270,457; 6,277,072; 6,280,380; 6,312,378; 6,336,903;6,358,203; 6,368,284; 6,398,728; and 6,440,066, which are herebyincorporated herein by reference.

As is illustrated in FIG. 9, the medical system 1000 may be used toimplement coordinated patient measuring and/or monitoring, diagnosis,and/or therapy in accordance with embodiments of the invention. Themedical system 1000 may include, for example, one or morepatient-internal medical devices 1010, such as a PIMD, and one or morepatient-external medical devices 1020, such as a monitor or signaldisplay device. Each of the patient-internal 1010 and patient-external1020 medical devices may include one or more of a patient monitoringunit 1012, 1022, a diagnostics unit 1014, 1024, and/or a therapy unit1016, 1026.

The patient-external medical device 1020 performs monitoring, and/ordiagnosis, and/or therapy functions external to the patient (i.e., notinvasively implanted within the patient's body). The patient-externalmedical device 1020 may be positioned on the patient, near the patient,or in any location external to the patient.

The patient-internal and patient-external medical devices 1010, 1020 maybe coupled to one or more sensors 1041, 1042, 1045, 1046, patientinput/trigger devices 1043, 1047, and/or other information acquisitiondevices 1044, 1048. The sensors 1041, 1042, 1045, 1046, patientinput/trigger devices 1043, 1047, and/or other information acquisitiondevices 1044, 1048 may be employed to detect conditions relevant to themonitoring, diagnostic, and/or therapeutic functions of thepatient-internal and patient-external medical devices 1010, 1020.

The medical devices 1010, 1020 may each be coupled to one or morepatient-internal sensors 1041, 1045 that are fully or partiallyimplantable within the patient. The medical devices 1010, 1020 may alsobe coupled to patient-external sensors positioned on, near, or in aremote location with respect to the patient. The patient-internal andpatient-exterhal sensors are used to sense conditions, such asphysiological or environmental conditions, that affect the patient.

The patient-internal sensors 1041 may be coupled to the patient-internalmedical device 1010 through one or more internal leads 1053. Stillreferring to FIG. 9, one or more patient-internal sensors 1041 may beequipped with transceiver circuitry to support wireless communicationsbetween the one or more patient-internal sensors 1041 and thepatient-internal medical device 1010 and/or the patient-external medicaldevice 1020.

The patient-external sensors 1042 may be coupled to the patient-internalmedical device 1010 and/or the patient-external medical device 1020through one or more internal leads 1055 or through wireless connections.Patient-external sensors 1042 may communicate with the patient-internalmedical device 1010 wirelessly. Patient-external sensors 1042 may becoupled to the patient-external medical device 1020 through one or moreinternal leads 1057 or through a wireless link.

In an embodiment of the present invention, the patient-external medicaldevice 1020 includes a visual display configured to simultaneouslydisplay non-electrophysiological signals and ECG signals. For example,the display may present the information visually. The patient-externalmedical device 1020 may also, or alternately, provide signals to othercomponents of the medical system 1000 for presentation to a clinician,whether local to the patient or remote to the patient.

Referring still to FIG. 9, the medical devices 1010, 1020 may beconnected to one or more information acquisition devices 1044, 1048,such as a database that stores information useful in connection with themonitoring, diagnostic, or therapy functions of the medical devices1010, 1020. For example, one or more of the medical devices 1010, 1020may be coupled through a network to a patient information server 1030.

The input/trigger devices 1043, 1047 are used to allow the physician,clinician, and/or patient to manually trigger and/or transferinformation to the medical devices 1010, 1020. The input/trigger devices1043, 1047 may be particularly useful for inputting informationconcerning patient perceptions, such as a perceived cardiac event, howwell the patient feels, and other information not automatically sensedor detected by the medical devices 1010, 1020. For example, the patientmay trigger the input/trigger device 1043 upon perceiving a cardiacevent. The trigger may then initiate the recording of cardiac signalsand/or other sensor signals in the patient-internal device 1010. Later,a clinician may trigger the input/trigger device 1047, initiating thetransfer of the recorded cardiac and/or other signals from thepatient-internal device 1010 to the patient-external device 1020 fordisplay and diagnosis.

In one embodiment, the patient-internal medical device 1010 and thepatient-external medical device 1020 may communicate through a wirelesslink between the medical devices 1010, 1020. For example, thepatient-internal and patient-external devices 1010, 1020 may be coupledthrough a short-range radio link, such as Bluetooth, IEEE 802.11, and/ora proprietary wireless protocol. The communications link may facilitateunidirectional or bi-directional communication between thepatient-internal 1010 and patient-external 1020 medical devices. Dataand/or control signals may be transmitted between the patient-internal1010 and patient-external 1020 medical devices to coordinate thefunctions of the medical devices 1010, 1020.

In another embodiment, patient data may be downloaded from one or moreof the medical devices periodically or on command, and stored at thepatient information server 1030. The physician and/or the patient maycommunicate with the medical devices and the patient information server1030, for example, to acquire patient data or to initiate, terminate, ormodify recording and/or therapy.

The data stored on the patient information server 1030 may be accessibleby the patient and the patient's physician through one or more terminals1050, e.g., remote computers located in the patient's home or thephysician's office. The patient information server 1030 may be used tocommunicate to one or more of the patient-internal and patient-externalmedical devices 1010, 1020 to provide remote control of the monitoring,diagnosis, and/or therapy functions of the medical devices 1010, 1020.

In one embodiment, the patient's physician may access patient datatransmitted from the medical devices 1010, 1020 to the patientinformation server 1030. After evaluation of the patient data, thepatient's physician may communicate with one or more of thepatient-internal or patient-external devices 1010, 1020 through an APMsystem 1040 to initiate, terminate, or modify the monitoring,diagnostic, and/or therapy functions of the patient-internal and/orpatient-external medical systems 1010, 1020.

In another embodiment, the patient-internal and patient-external medicaldevices 1010, 1020 may not communicate directly, but may communicateindirectly through the APM system 1040. In this embodiment, the APMsystem 1040 may operate as an intermediary between two or more of themedical devices 1010, 1020. For example, data and/or control informationmay be transferred from one of the medical devices 1010, 1020 to the APMsystem 1040. The APM system 1040 may transfer the data and/or controlinformation to another of the medical devices 1010, 1020.

In one embodiment, the APM system 1040 may communicate directly with thepatient-internal and/or patient-external medical devices 1010, 1020. Inanother embodiment, the APM system 1040 may communicate with thepatient-internal and/or patient-external medical devices 1010, 1020through medical device programmers 1060, 1070 respectively associatedwith each medical device 1010, 1020. As was stated previously, thepatient-internal medical device 1010 may take the form of an implantablePIMD.

A PIMD may operate in a batch mode or adaptively, allowing for on-lineor off-line implementation. To save power, the system may include theoption for a hierarchical decision-making routine that uses algorithmsknown in the art for identifying the presence of arrhythmias or noise inthe collected signal and initiating the methods of the presentinvention.

Various modifications and additions can be made to the preferredembodiments discussed hereinabove without departing from the scope ofthe present invention. Accordingly, the scope of the present inventionshould not be limited by the particular embodiments described above, butshould be defined only by the claims set forth below and equivalentsthereof.

1. A method of delivering a subcutaneous defibrillation therapy,comprising: selecting at least a pair of subcutaneous non-intrathoracicelectrodes from a plurality of subcutaneous non-intrathoracicelectrodes, the pair selected to provide an increased cardiacsignal-to-noise ratio relative to other pairs of the plurality ofsubcutaneous non-intrathoracic electrodes; sensing a fibrillation eventusing the selected pair of subcutaneous non-intrathoracic electrodes ofthe plurality of subcutaneous non-intrathoracic electrodes; determininga fibrillation cycle length associated with the fibrillation event; anddelivering, using one or more combinations of electrodes of theplurality of subcutaneous non-intrathoracic electrodes, a plurality ofdefibrillation pulses to treat the fibrillation event, wherein theelectrodes of each electrode combination are structurally supported in afixed relationship such that the electrodes of each respective electrodecombination are located extra-thoracically at opposing sides of theheart, and delivery of a latter pulse of the defibrillation pulsessubsequent to a former pulse of the defibrillation pulses is separatedin time by a first delay associated with the fibrillation cycle length.2. The method of claim 1, further comprising delivering, using at leastone of the combinations of electrodes, an additional defibrillationpulse following a second delay relative to delivery of the latterdefibrillation pulse, the second delay associated with a percentage ofthe fibrillation cycle length.
 3. The method of claim 2, wherein thefirst and second delays are associated with different percentages of thefibrillation cycle length.
 4. The method of claim 1, wherein the formerpulse is delivered from an electrode combination of the one or moreelectrode combinations different from that of the latter pulse.
 5. Themethod of claim 1, wherein the fibrillation event is sensed by theselected subcutaneous non-intrathoracic electrode pair corresponding toa first electrode combination of the one or more electrode combinationsand one or both of the former and latter pulses are delivered from asecond electrode combination of the one or more electrode combinations.6. The method of claim 1, wherein the fibrillation event comprises aventricular fibrillation event.
 7. The method of claim 1, wherein thefirst delay is greater than about 50 percent of the fibrillation cyclelength.
 8. The method of claim 1, wherein the first delay is greaterthan about 75 percent of the fibrillation cycle length.
 9. The method ofclaim 1, wherein the first delay is about 75 to 100 percent of thefibrillation cycle length.
 10. The method of claim 1, wherein thefibrillation cycle length is determined using a Fourier analysis of thefibrillation event.
 11. The method of claim 1, wherein the fibrillationcycle length is determined using an auto-correlation analysis of thefibrillation event.
 12. The method of claim 1, wherein the fibrillationcycle length is determined by counting signal features of thefibrillation event over a duration of time.
 13. The method of claim 1,wherein the former and latter pulses are substantially similar in termsof one or more of amplitude, duration, and phase.
 14. The method ofclaim 1, wherein the former and latter pulses are substantiallydifferent in terms of one or more of amplitude, duration, and phase. 15.The method of claim 1, wherein the former and latter pulses comprisemonophasic waveforms.
 16. The method of claim 1, wherein the former andlatter pulses comprise biphasic waveforms.
 17. The method of claim 1,wherein one of the former and latter pulses comprises a monophasicwaveform and the other of the former and latter pulses comprises abiphasic waveform.
 18. The method of claim 1, wherein the fibrillationevent comprises a ventricular tachycardia event.
 19. The method of claim1, wherein delivering the plurality of defibrillation pulses comprisesdelivering at least one pulse of the plurality of defibrillation pulsesusing a first electrode combination of the one or more combinations ofelectrodes, the first electrode combination comprising a superiorelectrode located superior relative to the heart and an inferiorelectrode located inferior relative to the heart.
 20. The method ofclaim 1, wherein delivering the plurality of defibrillation pulsescomprises delivering at least one pulse of the plurality ofdefibrillation pulses using a first electrode combination of the one ormore combinations of electrodes, the first electrode combinationcomprising an anterior electrode located anterior relative to the heartand a posterior electrode located posterior relative to the heart.
 21. Asystem for delivering a subcutaneous defibrillation therapy, comprising:a housing configured for subcutaneous non-intrathoracic placementrelative to a patient's heart; detection circuitry provided in thehousing, the detection circuitry configured to detect a fibrillationevent; energy delivery circuitry provided in the housing; a plurality ofcombinations of electrodes configured for subcutaneous non-intrathoracicplacement relative to the patient's heart, the electrodes of eachelectrode combination coupled to the detection and energy deliverycircuitry and structurally supported in a fixed relationship such thatthe electrodes of each electrode combination are locatedextra-thoracically at opposing sides of the heart; and a controllerprovided in the housing and coupled to the detection and energy deliverycircuitry, the controller configured to select a pair of subcutaneousnon-intrathoracic electrodes from the plurality of electrodecombinations based on an increased cardiac signal-to-noise ratioassociated with the selected pair of subcutaneous non-intrathoracicelectrodes relative to other pairs of the plurality of electrodecombinations and to determine a fibrillation cycle length associatedwith the fibrillation event using the selected pair of subcutaneousnon-intrathoracic electrodes, the controller coordinating delivery of alatter defibrillation pulse following a delay relative to delivery of aformer defibrillation pulse, the delay defining a percentage of thefibrillation cycle length.
 22. The system of claim 21, wherein thecontroller is configured to deliver an additional defibrillation pulsefollowing a second delay relative to delivery of the latterdefibrillation pulse, the second delay defining a percentage of thefibrillation cycle length.
 23. The system of claim 21, wherein theselected electrode combination is used for detection of the fibrillationevent and delivery of the former and latter defibrillation pulses. 24.The system of claim 21, wherein a first electrode combination of theplurality of electrode combinations is used for detection of thefibrillation event and a second electrode combination of the pluralityof electrode combinations is used for delivery of the former and latterdefibrillation pulses.
 25. The system of claim 21, wherein a firstelectrode combination of the plurality of electrode combinations is usedfor delivery of the former defibrillation pulse and a second electrodecombination of the plurality of electrode combinations is used fordelivery of the latter defibrillation pulse.
 26. The system of claim 21,wherein the delay is greater than about 50 percent of the fibrillationcycle length.
 27. The system of claim 21, wherein the delay is greaterthan about 75 percent of the fibrillation cycle length.
 28. The systemof claim 19, wherein the delay is about 75 to 100 percent of thefibrillation cycle length.
 29. The system of claim 21, wherein thecontroller is configured to determine the fibrillation cycle lengthusing a Fourier analysis of the fibrillation event.
 30. The system ofclaim 21, wherein the controller is configured to determine thefibrillation cycle length using an auto-correlation analysis of thefibrillation event.
 31. The system of claim 21, wherein the controlleris configured to determine the fibrillation cycle length by countingsignal features of the fibrillation event over a duration of time. 32.The system of claim 21, wherein the former and latter defibrillationpulses are substantially similar in terms of one or more of amplitude,duration, and phase.
 33. The system of claim 21, wherein the former andlatter defibrillation pulses are substantially different in terms of oneor more of amplitude, duration, and phase.
 34. The system of claim 21,wherein the former and latter defibrillation pulses comprise monophasicwaveforms.
 35. The system of claim 21, wherein the former and latterdefibrillation pulses comprise biphasic waveforms.
 36. The system ofclaim 21, wherein one of the former and latter defibrillation pulsescomprises a monophasic waveform and the other of the former and latterdefibrillation pulses comprises a biphasic waveform.
 37. The system ofclaim 21, wherein the fibrillation event comprises a ventriculartachycardia event or a ventricular fibrillation event.
 38. The system ofclaim 21, further comprising a first electrode combination of theplurality of combinations of electrodes, the first electrode combinationcomprising a superior electrode located superior relative to the heartand an inferior electrode located inferior relative to the heart. 39.The system of claim 21, further comprising a first electrode combinationof the plurality of combinations of electrodes, the first electrodecombination comprising an anterior electrode located anterior relativeto the heart and a posterior electrode located posterior relative to theheart.
 40. A system for delivering a subcutaneous defibrillationtherapy, comprising: means for selecting a pair of subcutaneousnon-intrathoracic electrodes from a plurality of subcutaneousnon-intrathoracic electrodes, the pair selected to provide an increasedcardiac signal-to-noise ratio relative to other pairs of the pluralityof subcutaneous non-intrathoracic electrodes; means for sensing afibrillation event using the selected pair of subcutaneousnon-intrathoracic electrodes of the plurality of subcutaneousnon-intrathoracic electrodes; means for determining a fibrillation cyclelength associated with the fibrillation event; means for delivering,using a first combination of electrodes of the plurality of subcutaneousnon-intrathoracic electrodes, a former defibrillation pulse to treat thefibrillation event; means for delivering, using the first combination ora second combination of electrodes of the plurality of subcutaneousnon-intrathoracic electrodes, a latter defibrillation pulse following adelay relative to delivery of the former defibrillation pulse to treatthe fibrillation event, the delay associated with a percentage of thefibrillation cycle length; and means for structurally supporting theelectrodes of each of the first and the second electrode combinations ina fixed relationship such that the electrodes of each respectiveelectrode combination are located extra-thoracically at opposing sidesof the heart.